When manufacturing integrated circuits, various needs exist for testing the devices being fabricated. Such devices often must be tested not only to determine whether they are operative, but also to determine their actual specifications. The test results are used to separate good devices from bad devices, to establish component grading, and to establish a test record for devices required to carry such a pedigree (e.g., for certain military, space and atomic energy applications).
As an example, consider a typical voltage reference product (e.g., for a digital-to-analog converter) whose temperature coefficient must be determined during the test process. One prior art method for testing electronic devices such as voltage references is to test a number of such references in sequence, in an assembly-line fashion. The references are arranged on or supplied via an apparatus which sequentially moves each of the references near an automatic tester which tests the references one at a time, performing all necessary tests on one reference before advancing to test the next reference. The measurements may be stored in the tester or in a computer, where they may be filed under the sequential number of the reference. For instance, the first reference tested is assigned the identification number 1, the second reference tested is assigned the identification number 2, and so forth, for all of the references. In calculating temperature coefficients, the output voltage of each reference is measured at a number of different temperatures, and these temperatures are also stored. The temperature coefficients for the references can then be calculated. This temperature coefficient is then compared to a specified value, to determine whether the reference has passed the test, or to a group of values, to grade the voltage reference by quality.
The problem with this method of testing is that it is very slow. The device under test (DUT) must be brought to each test temperature, in sequence, and allowed to stabilize before each measurement is made. Additionally, the test jig itself must be allowed to stabilize at each test temperature, and it may exhibit considerable thermal inertia.
Another prior art method for testing electronic devices for temperature coefficient is to arrange all of the DUTs on a large board. The board with the DUTs is then placed into an environmental chamber, where the testing takes place. The main problem with this method is that it is expensive to use and maintain an environmental chamber to perform the testing.
Yet another approach is to bring the test jig to a selected test temperature, check each DUT at that temperature, in sequence, recording the test data for each DUT as it is measured. Then, the jig temperature is changed to the next test temperature and all the DUT's are tested as before, at this new temperature. The assumption is made that the DUT's remained in the same sequence, so the second round of test data will be recorded for the correct DUT's. This sequence is repeated as many times as there are test temperatures. Once the data is all available, the temperature coefficients are calculated.
If the references do not remain in sequence when they are being tested, then the test data is meaningless. In practice it is very difficult to maintain the references in sequence. For example, device handlers may become jammed and DUT's may be damaged or otherwise may have to be removed from the sequence. This will necessitate that the tests be conducted again, from the start.
A solution to these problems is one that allows the tester to individually recognize each DUT. Each DUT is given a unique identification number and the test data can be indexed according to the DUT. There have been several methods utilized in the past for associating the identification number with the DUT, such as affixing or printing a bar code on the underside of the DUT package and mounting in the test jig an optical reader for reading the bar code. The problem with such methods is that they add considerably expense. For example, a bar code identification system requires either manual labor or additional apparatus for attaching or printing a bar code on the DUT, and equipment (i.e., an optical scanning head) to read the code.
It is therefore an object of the present invention to provide an improved and inexpensive method for identifying integrated circuit devices which are to be tested.
Another object of the invention is to provide a method for identifying integrated circuit devices which are to be tested at varying temperatures, to allow flexibility in test sequencing.
Still another object of the invention is to provide an integrated circuit structure with means facilitating identification of a DUT during the testing process.